Advances in semiconductor processing have resulted in the drastic reduction in device sizes. The gate length of a transistor, meal line widths and spacings, and oxide thicknesses have all undergone a dramatic reduction. However, the voltages applies to these reduced-size transistors must also be reduced to prevent damage to these increasingly tiny devices. Thus the power-supply voltage, such as Vcc or Vdd, has also been reduced from 5 volts to 3.3 volts to 1.8 volts, and now to 1.5 or even 1.2 volts.
While the device sizes have scaled downward, the transistor threshold voltage, Vtn or Vtp, has not scaled down as dramatically. This threshold voltage is now closer to the power-supply voltage than ever before. Designing circuits for low power-supply voltages while the threshold voltages remain around 0.7 volts is quite challenging.
A widely used circuit is the oscillator. An oscillator produces an output that oscillates, usually between the power supply voltage and ground when the loading on the output is not too severe. Oscillators are widely used as part of phase-locked loops (PLL's), clock generators, voltage pumps, and back-bias generator circuits.
A basic oscillator can be constructed from a loop or ring of an odd number of inverters. Such ring oscillators have a period or cycle time determined by the delay around the loop. Other oscillators use R-C delays to set cycle times.
The duty cycle or percent of the period that the output is high rather than low is another parameter of an oscillator circuit. 50%-50% duty cycles may be required in some applications, while other applications may need a skewed clock such as 60%-40%.
The amount of time that an oscillator needs to begin oscillating is also determined by the circuit used, as well as the ramp time of the power supply. Oscillators that can begin oscillating at low power supply voltages are desirable since the clocks may begin clocking during reset before the power supply has reached its full steady-state operating value. These extra clock pulses during reset are especially valuable for large systems with many clocked nodes, since the added clocking can help reset the circuit into a know stable state that consumes less power than if nodes remain floating after reset.
What is desired is an oscillator circuit that operates at very low power-supply voltages. An oscillator that quickly begins oscillating during reset is also desirable. An oscillator that has a duty cycle that can be set by circuit parameters is also desirable.